1. Field of the Invention
The present invention is in the field of machines and methods used to service automotive power steering systems. More particularly, the present invention relates to a machine and method for removing the old and contaminated power steering fluid from an automotive power steering system, while simultaneously replacing the old fluid with new fluid as the automotive vehicle runs to activate the power steering system. The machine and method according to the present invention are fail-safe, such that damage to the power steering system from running dry cannot happen even from inattention or neglect on the part of a service technician.
2. Related Technology
A conventional machine for service of the power steering system of an automobile or other automotive vehicle is known in accord with U.S. Pat. No. 5,415,247, issued May 16, 1995 to Robert Knorr. The '247 patent is believed to teach a power steering fluid exchange machine for use with the power steering of an automotive vehicle, such as a truck or automobile. The Knorr '247 patent appears to utilize two electric motor driven pump units which are operated simultaneously, one withdrawing old power steering fluid from the reservoir of a power steering system, and the other simultaneously delivering new power steering fluid from a supply into the reservoir of the power steering system. The motor driven pump units are powered by electrical power from the vehicle under service, and the engine of the vehicle under service is running with the steering turned to one extreme of its motion, assertedly to open the valves of the power steering system.
Unfortunately, the service machine according to the '247 patent has a serious shortcoming, and the explanation of its use in cooperation with the power steering system of a vehicle while the vehicle is operating evidences a lack of understanding about the workings of automotive power steering systems. That is, the Knorr '247 patent specifically teaches and claims a power steering service machine of the type described in which a first conduit delivers fresh power steering fluid into an upper part of the reservoir of the power steering system (i.e., at a first level). A second conduit extends into the reservoir to a level below the first level, and withdraws old power steering fluid from the reservoir of the system while the engine of the vehicle is running.
However, in the event that the service technician allows the supply of new power steering fluid to run out while the service procedure is in process, and does not stop the process before the service machine withdraws substantially all of the fluid from the power steering system, then the power steering pump of the vehicle may be seriously damaged or destroyed by running dry of power steering fluid. This is clearly the case because the Knorr '247 patent teaches to withdraw the old fluid (i.e., any fluid in the reservoir of the power steering system) at a low level, and to deliver the new fluid at a higher level. Ostensibly, this difference in deliver and withdrawal levels for the fluid minimized admixture of the new fluid with the old and effects an economy in the amount of new power steering fluid needed to effect a service for a vehicle. However, the risk of damage to the power steering pump of a vehicle being serviced is too great, and the cost of power steering fluid is minimal in comparison to the cost of replacing a damaged power steering pump.
Still further, the Knorr '247 patent teaches that during a service procedure for a power steering system, the machine taught by this patent is used, and the steering of the vehicle under service is turned to one extreme or the other while the engine of the vehicle runs. This turning of the vehicle steering to one extreme is said to open the valves of the power steering system and allow substantially all of the old fluid to be flushed from the system. Knorr recommends use of a strap or belt to hold the vehicle's steering wheel in the extreme right ward position for this purpose.
Unfortunately, the explanation provided by Knorr is not a true or complete representation of the operation of an automotive power steering system. That is, automotive power steering systems of the common type found on automobiles, for example, are open-centered hydraulic servo systems operating in conjunction with a mechanical steering gear which provides a positive driving engagement between the steering wheel of the vehicle and the steering linkage moving the dirigible wheels of the vehicle for steering movements. A power steering pump of the vehicle is operated by the vehicle engine when this engine is running, and draws power steering fluid at ambient pressure from a reservoir, pressurizes this fluid, and sends the fluid to an open-centered hydraulic servo control valve of the power steering system.
The open-centered hydraulic servo control valve is associated with the steering wheel of the vehicle by connection with a steering input shaft, and receives steering input movements from the steering wheel as these movements are transferred to the steering linkage of the vehicle. This control valve moves from an open-centered position to a partially closed/partially opened position in response to steering torque at the steering wheel reaching a certain threshold.
The servo control valve is associated with a hydraulic actuator, which is double acting so as to provide an assist in driving the dirigible wheels toward a steering position selected by the position of the steering wheel of the vehicle. When the open-centered servo control valve is in its centered position, pressurized power steering fluid from the pump is communicated equally to both sides of the double acting actuator, providing a balanced force that does not effect a steering assist force. In this position of the control valve, fluid flows from the pump to the control valve, is throttled to a lower pressure, and returns to the reservoir of the pump. There is little of no actual flow of fluid from the control valve to or from either one of the sides of the double acting actuator.
On the other hand, when the control valve is moved, by a steering input applied at the steering wheel, for example, to a partially closed/partially open position, communication of pressurized fluid from the pump is increased to one side of the double-sided actuator, while communication of the fluid from the other side of the actuator to the fluid reservoir is increased. As a result, the actuator provides a force and movement in one steering direction, with the pressurized side of the actuator receiving pressurized fluid, and the movement of the actuator displacing fluid from the low-pressure side of the actuator to the fluid reservoir of the power steering system. Thus, the expedient recommended by Knorr (keeping the steering wheel in one extreme of its position, would result in the actuator of the system not being flushed of old power steering fluid, and in this retained old fluid contaminating the new fluid as soon as the system is returned to service.
In order to remove the old fluid from a power steering system and replace this old fluid with new fluid, the actuator of the system must be exercised through its full movement as new fluid is supplied by the pump of the system to this actuator, and as the old fluid returned from the actuator to the fluid reservoir is removed.